Vertical batch furnace assembly

ABSTRACT

A vertical batch furnace assembly for processing wafers comprising a cassette handling space, a wafer handling space, and a first wall separating the cassette handling space from the wafer handling space. The wall having a wafer transfer opening. The wafer transfer opening is associated with a cassette carrousel comprising a carrousel stage having a plurality of cassette support surfaces each configured for supporting a wafer cassette. The carrousel stage is rotatable by an actuator around a substantially vertical axis to transfer each cassette support surface to a wafer transfer position in front of the wafer transfer opening and to at least one cassette load/retrieve position, wherein the vertical batch furnace assembly is configured to load or retrieve a wafer cassette on or from a cassette support surface of the carousel stage which is in the at least one load/retrieve position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/880,850 filed Jul. 31, 2019 titled VERTICAL BATCH FURNACE ASSEMBLY, the disclosure of which is hereby incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to a vertical batch furnace assembly for processing wafer.

BACKGROUND

Vertical batch furnace assemblies for processing wafers comprise a cassette handling space, a wafer handling space, and a wall separating these two spaces. The wall has a wafer transfer opening provided with a cassette door opener device in front of which a wafer transfer position for a wafer cassette is provided. The cassette handling space is provided with cassette handling mechanism which may transport wafer cassettes between the wafer transfer position and a cassette storage in which the wafer cassettes may be stored. The cassette handling space may also be provided with a transfer mechanism to move a wafer cassette from the wafer transfer position towards the cassette door opener device, so that the cassette door opener device may engage the wafer cassette and open the cassette lid of the wafer cassette. The wafer handling space of the vertical batch furnace assembly may also be provided with a wafer handler which may remove wafers from a wafer cassette which is engaged to the cassette door opener device, and transfer those wafers through the transfer opening to a wafer boat for applying a treatment in a process chamber. After treatment of said wafers the wafer handler may also put the wafers back into the wafer cassette which is engaged to the cassette door opener device. The cassette door opener device may release the engaged wafer cassette and the transfer mechanism may move the wafer cassette back to the wafer transfer position. Subsequently the cassette handling mechanism may transfer the wafer cassette to the cassette storage and replace it with another wafer cassette from the cassette storage.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form. These concepts are described in further detail in the detailed description of example embodiments of the disclosure below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

It may be realized that typically multiple wafer cassettes need to be unloaded to fill an entire wafer boat. That during the transporting of wafer cassettes to and from the wafer transfer position, the wafer handler may be unable to transfer wafers and remains idle. That during an engagement of a wafer cassette with the cassette door opener device, the cassette handling mechanism may be unable to transport a wafer cassette to or from the wafer transfer position and remains idle. That this idle time may be detrimental to the efficiency of the vertical batch furnace assembly.

Therefore, it may be an object to provide a vertical batch furnace assembly with improved wafer cassette handling.

To that end, there may be provided a vertical batch furnace assembly according to claim 1. More particularly, there may be provided a vertical batch furnace assembly for processing wafers. The vertical batch furnace assembly may comprise a cassette handling space, a wafer handling space, and a first wall. The first wall may separate the cassette handling space from the wafer handling space and may have at least one wafer transfer opening. In front of the at least one wafer transfer opening, at a side of the first wall which is directed to the cassette handling space, a wafer transfer position for a wafer cassette may be provided. The at least one wafer transfer opening may be associated with at least one cassette carrousel. The at least one cassette carrousel may comprise a carrousel stage which may have a plurality of cassette support surfaces which each may be configured for supporting a wafer cassette. The carrousel stage may be rotatable by an actuator around a substantially vertical axis to transfer each cassette support surface to at least one cassette load/retrieve position, and to the wafer transfer position in front of the wafer transfer opening. The vertical batch furnace assembly may be configured to load or retrieve a wafer cassette on or from a cassette support surface of the carousel stage which is in the at least one cassette load/retrieve position.

For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught or suggested herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

Various embodiments are claimed in the dependent claims, which will be further elucidated with reference to an example shown in the figures. The embodiments may be combined or may be applied separate from each other.

All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments will become readily apparent to those skilled in the art from the following detailed description of certain embodiments having reference to the attached figures, the invention not being limited to any particular embodiment(s) disclosed.

BRIEF DESCRIPTION OF THE FIGURES

While the specification concludes with claims particularly pointing out and distinctly claiming what are regarded as embodiments of the invention, the advantages of embodiments of the disclosure may be more readily ascertained from the description of certain examples of the embodiments of the disclosure when read in conjunction with the accompanying drawings, in which:

FIG. 1 shows a top, sectional view of a cassette handling space of an example of a vertical batch furnace assembly according to the description; and

FIG. 2 shows a side, sectional view of the example of FIG. 1.

DETAILED DESCRIPTION OF THE FIGURES

In this application similar or corresponding features are denoted by similar or corresponding reference signs. The description of the various embodiments is not limited to the example shown in the figures and the reference numbers used in the detailed description and the claims are not intended to limit the description of the embodiments, but are included to elucidate the embodiments by referring to the example shown in the figures.

Although certain embodiments and examples are disclosed below, it will be understood by those in the art that the invention extends beyond the specifically disclosed embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention disclosed should not be limited by the particular disclosed embodiments described below. The illustrations presented herein are not meant to be actual views of any particular material, structure, or device, but are merely idealized representations that are used to describe embodiments of the disclosure.

As used herein, the term “wafer” may refer to any underlying material or materials that may be used, or upon which, a device, a circuit, or a film may be formed.

In the most general term the present disclosure may provide a vertical batch furnace assembly 10 for processing wafers, as illustrated in FIGS. 1 and 2. The vertical batch furnace assembly 10 may comprise a cassette handling space 12, a wafer handling space 14, and a first wall 16. The first wall 16 may separate the cassette handling space 12 from the wafer handling space 14 and may have at least one wafer transfer opening 18, 20. In front of the at least one wafer transfer opening 18, 20, at a side of the first wall 16 which is directed to the cassette handling space 12, a wafer transfer position 22, 24 for a wafer cassette 26 may be provided. The at least one wafer transfer opening 18, 20 may be associated with at least one cassette carrousel 28, 30. The at least one cassette carrousel 28, 30 may comprise a carrousel stage 32 having a plurality of cassette support surfaces 34 which each may be configured for supporting a wafer cassette 26. The carrousel stage 32 may be rotatable by an actuator around a substantially vertical axis 36 to transfer each cassette support surface 34 to at least one cassette load/retrieve position 38, 40, and to the wafer transfer position 22, 24 in front of the wafer transfer opening 18, 20. The vertical batch furnace assembly, in particular a cassette handling mechanism, may be configured to load or retrieve a wafer cassette 26 on or from a cassette support surface 34 of the carousel stage 32 which is in the at least one load/retrieve position 38,40.

By virtue of the at least one cassette carrousel 28, 30 according to the description, the position in which the cassette handling mechanism 44, 46 places a wafer cassette 26 on the carrousel stage 32, and the position from which wafers are transferred to the wafer boat, are different positions. This means that the vertical batch furnace assembly 10, in particular a wafer handler thereof, may transfer wafers from the wafer cassette 26 which is placed in the wafer transfer position 22, 24, and that the vertical batch furnace assembly 10, in particular a cassette handler mechanism 44, 46 thereof, may simultaneously transfer another wafer cassette 26 to the load/retrieve position 38, 40. Opening or closing of wafer cassettes 26 and loading/unloading of the wafers may only have to stop whilst the carousel stage 32 is rotated by the actuator. This may be a much smaller time than the time which may typically be needed to transfer a wafer cassette 26 with a cassette handling mechanism 44, 46 from the wafer transfer position 22, 24 to the cassette storage and transferring a new wafer cassette 26 vice versa. Thus, this solution may be beneficial relative to a solution in which the handling speed of the cassette handling mechanism 44, 46 is simply increased. Handling wafer cassettes 26 with high speed may cause all kinds of other problems, such as increased risk of particle release which may lead to a higher rejection percentage of the wafers being produced. Such problems may be reduced because the cassette handling mechanism 44, 46 may transfer a wafer cassette 26 to and/or from the cassette load/retrieve position 38, 40 while at the same time wafers are being transferred to the wafer boat. This means that the cassette handling mechanism 44, 46 may take more time for this transferal and that the cassette handling mechanism 44, 46 may move relatively slow and thus smoothly. Thus, the cassette handling mechanism 44, 46 may be relatively simple and thus less expensive and wafer cassettes 26 are transferred smoothly so that the release of particles may be reduced.

Another advantage of this configuration may be that the cassettes, after the wafers have been transferred by the wafer handler to the wafer boat for processing, can be left empty on the carrousel stage(s). So, when the next action is to unload a processed wafer boat the empty cassettes are already there on the carrousel stage(s). By leaving the empty cassette(s) on the carrousel stage(s), the “work” to be done by the cassette handler is cut in half because it doesn't have to move and store empty cassettes. If this is done on two carrousel stages, 6 cassettes can be used in this way, which corresponds to a pretty full boat with 150 wafers.

Yet another advantage is that in the case of filler or dummy wafers the cassette for the filler or dummy wafers can remain on the carrousel stage all the time while the filler wafers transfer between the filler cassette and the boat continuously.

In an embodiment each carrousel stage 32 may comprise three cassette support surfaces 34. The at least one cassette load/receive position 38, 40 may comprises a first load/retrieve position 38 and a second load/retrieve position 40. The carrousel stage 32 may be rotatable to transfer each of the three cassette support surfaces 34 to the first load/retrieve position 38, the second load/retrieve position 40, and the wafer transfer position 22, 24.

By having a first load/retrieve position 38 and a second load/retrieve position 40, multiple wafer cassettes 26 may be placed on the carousel stage 32 simultaneously. This may ensure that there is always a wafer cassette 26 on the carousel stage 32 available which may be rotated to the wafer transfer position 22, 24.

In an embodiment the at least one wafer transfer opening 18, 20 comprises a first wafer transfer opening 18 in front of which a first wafer transfer position 22 is provided, and a second wafer transfer opening 20 in front of which a second wafer transfer position 24 is provided.

The at least one cassette carrousel 28, 30 may comprise a first cassette carrousel 28 and a second cassette carrousel 30. The first cassette carrousel 28 may be associated with the first wafer transfer opening 18, and the second cassette carrousel 30 may be associated with the second wafer transfer opening 20. The first and second cassette carrousels 28, 30 may have co-linear vertical axes.

The cassette handling space 12 may comprise a cassette storage. The cassette storage may have a plurality of cassette storage positions 42 and may be configured to store a plurality of wafer cassettes 26. The cassette storage may, for example, comprise a number of racks which each have a number of cassette storage positions 42, for example above one and another and or next to one another. Alternatively or additionally, the cassette storage may comprise storage carrousels with storage carrousel stages on which cassette storage positions 42 are provided. The first cassette carrousel 28 and the second cassette carrousel 30 may be vertically spaced apart. The cassette storage may comprise at least one cassette storage carrousel in between the first cassette carrousel 28 and the second cassette carrousel 30.

By having a second wafer transfer opening 20 in addition to the first wafer transfer opening 18, wafers may be transferred trough both openings to a wafer boat. This will increase the throughput of the vertical batch furnace assembly and thus its efficiency. By having a cassette carrousel 28, 30 associated with each wafer transfer opening 18, 20 the advantages of the at least one cassette carrousel 28, 30 apply for each transfer opening 18, 20, making the vertical batch furnace assembly still more efficient. By having cassette storage positions 42 in between the first cassette carrousel 28 and the second cassette carrousel 30 optimal use is made of all available space in the cassette handling space 12.

In an embodiment the vertical batch furnace assembly 10 may further be provided with a blower comprising an elongated, slit-like nozzle substantially spanning a height of a cassette lid of a wafer cassette 26 when supported on a support surface 34 of the carrousel stage 32 of the at least one cassette carrousel 28, 30. The blower may be configured to blow a curtain-shaped jet stream of a gas substantially spanning the height of the cassette lid onto the cassette lid when the at least one cassette carrousel 28, 30 transfers the wafer cassette 26 from the at least one cassette load/receive position 38, 40 to the wafer transfer position 22, 24. The blower may be provided near the at least one cassette carrousel 28, 30. The blower may be mounted to the first wall 16.

By blowing the gas onto the cassette lid, any contamination or debris on the cassette lid may be blown away. By using a jet stream it may be ensured that a powerful gust of air reaches the cassette lid, thereby ensuring the contamination/debris to be blown away. Because the blower blows a curtain-shaped jet stream which substantially spans the height of the cassette lid, the entire height cassette lid is thus bereft of contamination/debris. When the cassette carrousel 28, 30 transfers the wafer cassette 26 from the at least one cassette load/receive position 38, 40 to the wafer transfer position 22, 24 the entire width of the cassette lid is transported past the blower. This means that the blower also blows away contamination/debris across the entire width of the cassette lid.

In an embodiment, the cassette storage, which may comprise storage racks and/or storage carrousels, may have 10 to 50 cassette storage positions 42. The cassette storage may e.g., have at least 40 cassette storage positions 42.

Typically about 50 to 250 wafers are needed to fill a wafer boat. Typically there are about 25 wafers stored in a wafer cassette. This means that typically 2 to 10 wafer cassettes are needed to fill one wafer boat. By having at least 40 cassette storage positions 42, enough wafer cassettes 26 may be stored to fill at least two wafer boats, in particular approximately four wafer boats. Thus the loading and unloading to and from the cassette handling space 12 can be postponed for quite some time while the vertical batch furnace is processing the wafers from the cassettes which are stored in the cassette handling space 12.

In an embodiment the wafer cassettes 26 may be embodied as front opening unified pods (FOUP's). FOUP's are considered standard enclosures for wafers in the industry. Using FOUP's is advantageous for exchangeability of the wafer cassettes between the vertical batch furnace assembly 10 and other wafer processing machines.

In an embodiment the vertical batch furnace assembly 10 may further comprise at least one cassette in-out port 48, and a cassette handler mechanism 44, 46. The at least one cassette in-out port 48 provides the entrance to and the exit from the vertical batch furnace assembly 10, for the exchange of wafer cassettes between the vertical batch furnace assembly 10 and the outside world. The at least one cassette in-out port 48 may be provided in a second wall 58 bounding the cassette handling space 12. The cassette handling mechanism 44, 46 may be configured like the other cassette handling mechanisms 44, 46 as described above. The cassette handler mechanism 44, 46 may be configured to transfer wafer cassettes 26 to and from the at least one cassette in-out port 48. Such a cassette handler mechanism 44, 46 may handle all of the wafer cassette transfers within the cassette handling space 12. This may provide an efficient and cost-effective way to transfer wafer cassettes 26.

In an embodiment, the cassette handling mechanism 44, 46 may comprise a first cassette handler 44 and a second cassette handler 46. The first cassette handler 44 may be configured to place a wafer cassette 26 onto a cassette support surface 34 of the carrousel stage 32 which is in the first load/retrieve position 38 and to retrieve the wafer cassette 26 from the cassette support surface 34 of the carrousel stage 32 which is in the first load/retrieve position 38. The second cassette handler 46 may be configured to place a wafer cassette 26 onto a cassette support surface 34 of the carrousel stage 32 which is in the second load/retrieve position 40 and to retrieve the wafer cassette 26 from the cassette support surface 34 of the carrousel stage 32 which is in the second load/retrieve position 40.

Alternatively, both the first cassette handler 44 and the second cassette handler 46 may be configured to place a wafer cassette 26 onto a cassette support surface 34 of the carrousel stage 32 which is in the first load/retrieve position 38 and to retrieve the wafer cassette 26 from the cassette support surface 34 of the carrousel stage 32 which is in the first load/retrieve position 38, as well as to place a wafer cassette 26 onto a cassette support surface 34 of the carrousel stage 32 which is in the second load/retrieve position 40 and to retrieve the wafer cassette 26 from the cassette support surface 34 of the carrousel stage 32 which is in the second load/retrieve position 40.

Each cassette handler 44, 46 may be provided with a cassette handler arm 50, and an elevator mechanism 52 which may be configured to reach cassette storage positions 42 at different vertical heights within the cassette handling space 12.

The elevator mechanism 52 of the first cassette handler 44 and the elevator mechanism 52 of the second cassette handler 46 may be arranged on or adjacent respectively a third wall 60 and a fourth wall 62 which bound the cassette handling space 12 on opposite ends.

By having two cassette handlers 44, 46, each associated with the first or second load/retrieve positions 38, 40, both cassette handlers 44, 46 may simultaneously transfer wafer cassettes 26 to and from the at least one cassette carrousel 28,30. This means that wafer cassettes 26 may be transferred more quickly from the cassette storage, which means an improvement of efficiency. By arranging the elevator mechanism 52 of the first cassette handler 44 opposite the elevator mechanism 52 of the second cassette handler 46, the arms 50 of both cassette handlers 44, 46 may cover different parts of the cassette handling space 12. Both may operate simultaneously without getting in each other's way. Moreover, both may be equipped with a relatively short arm 50 while, in combination still being able to cover the entire cassette handling space 12.

In an embodiment the at least one cassette carrousel 28, 30 may be provided with a transfer mechanism 54. The transfer mechanism 54 may be configured to move a wafer cassette 26 supported on a cassette support surface 34 in the wafer transfer position 22, 24 towards and/or away from the first wall 16 having the at least one wafer transfer opening 18, 20.

For each wafer transfer opening 18, 20 the vertical batch furnace assembly 10 may comprise a cassette door opener device 56. The cassette door opener device 56 may be arranged on the first wall 16 and may be configured to engage a wafer cassette 26 which is being moved towards the first wall 16 by the transfer mechanism 54 and to open a cassette door of said engaged wafer cassette 26.

The vertical batch furnace assembly 10 may further comprise a wafer handler, a process chamber, and a wafer boat handling device. The wafer handler may be positioned in the wafer handling space 14 and may be configured to transfer wafers between a wafer cassette 26 engaged to the cassette opener device 56 and a wafer boat. The process chamber may be configured to process wafers accommodated in the wafer boat. The wafer boat handling device may be positioned under the process chamber and adjacent the wafer handling space 14. The wafer boat handling device may be provided with the wafer boat in a wafer boat transfer position, and may be configured to transport the wafer boat to the process chamber, and to the wafer boat transfer position.

The at least one cassette carrousel 28, 30 may rotate a wafer cassette 26 to the wafer transfer position 22, 24 which is in front of the at least one wafer transfer opening 18, 20. By virtue of the transfer mechanism 54 the wafer cassette 26 may be moved towards and/or away from the cassette door opener device 56 arranged on the first wall, such that wafers in the wafer cassette 26 may be loaded to/unloaded from the wafer cassette 26 by the wafer handler.

Although illustrative embodiments of the present invention have been described above, in part with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this description are not necessarily all referring to the same embodiment.

Furthermore, it is noted that particular features, structures, or characteristics of one or more of the various embodiments which are described above may be used implemented independently from one another and may be combined in any suitable manner to form new, not explicitly described embodiments. The reference numbers used in the detailed description and the claims do not limit the description of the embodiments nor do they limit the claims. The reference numbers are solely used to clarify.

LEGEND

-   10—vertical batch furnace assembly -   12—cassette handling space -   14—wafer handling space -   16—first wall -   18—first wafer transfer opening -   20—second wafer transfer opening -   22—first wafer transfer position -   24—second wafer transfer position -   26—wafer cassette -   28—first cassette carrousel -   30—second cassette carrousel -   32—carrousel stage -   34—cassette support surface -   36—vertical axis -   38—first load/retrieve position -   40—second load/retrieve position -   42—cassette storage position -   44—first cassette handler -   46—second cassette handler -   48—cassette in-out port -   50—cassette handler arm -   52—elevator mechanism -   54—transfer mechanism -   56—cassette door opener device -   58—second wall -   60—third wall -   62—fourth wall 

1. A vertical batch furnace assembly for processing wafers comprising: a cassette handling space; a wafer handling space; and a first wall separating the cassette handling space from the wafer handling space and having at least one wafer transfer opening in front of which, at a side of the first wall which is directed to the cassette handling space, a wafer transfer position for a wafer cassette is provided; wherein the at least one wafer transfer opening is associated with at least one cassette carrousel comprising a carrousel stage having a plurality of cassette support surfaces each configured for supporting a wafer cassette, wherein the carrousel stage is rotatable by an actuator around a substantially vertical axis to transfer each cassette support surface to: at least one cassette load/retrieve position, wherein the vertical batch furnace assembly is configured to load or retrieve a wafer cassette on or from a cassette support surface of the carousel stage which is in the at least one load/retrieve position; and to the wafer transfer position in front of the wafer transfer opening.
 2. The vertical batch furnace assembly according to claim 1, wherein each carrousel stage comprises three cassette support surfaces, wherein the at least one cassette load/receive position comprises a first load/retrieve position and a second load/retrieve position, wherein the carrousel stage is rotatable to transfer each of the three cassette support surfaces to the first load/retrieve position, the second load/retrieve position, and the wafer transfer position.
 3. The vertical batch furnace assembly according to claim 1, wherein the at least one wafer transfer opening comprises a first wafer transfer opening in front of which a first wafer transfer position is provided, and a second wafer transfer opening in front of which a second wafer transfer position is provided.
 4. The vertical batch furnace assembly according to claim 3, wherein the at least one cassette carrousel comprises a first cassette carrousel and a second cassette carrousel, wherein the first cassette carrousel is associated with the first wafer transfer opening, and the second cassette carrousel is associated with the second wafer transfer opening, and wherein the first and second cassette carrousels have co-linear vertical axes.
 5. The vertical batch furnace assembly according to claim 4, wherein the cassette handling space comprises a cassette storage having a plurality of cassette storage positions and configured to store a plurality of wafer cassettes, wherein the first cassette carrousel and the second cassette carrousel are vertically spaced part, and wherein the cassette storage comprises cassette storage positions in between the first cassette carrousel and the second cassette carrousel.
 6. The vertical batch furnace assembly according claim 1, further provided with a blower comprising an elongated, slit-like nozzle substantially spanning a height of a cassette lid of a wafer cassette when supported on a support surface of the carrousel stage of the at least one cassette carrousel, wherein the blower is configured to blow a curtain-shaped jet stream of a gas substantially spanning the height of the cassette lid onto the cassette lid when the at least one cassette carrousel transfers the wafer cassette from the at least one cassette load/receive position to the wafer transfer position.
 7. The vertical batch furnace assembly according to claim 6, wherein the blower is provided near the at least one cassette carrousel.
 8. The vertical batch furnace assembly according to claim 1, wherein the cassette handling space comprises: a cassette storage having a plurality of cassette storage positions and configured to store a plurality of wafer cassettes; and a cassette handling mechanism configured to transfer wafer cassettes within the cassette handling space to and from the cassette storage positions, wherein the cassette storage has 10 to 50 cassette storage positions.
 9. The vertical batch furnace assembly according to claim 1, wherein the wafer cassettes are embodied as front opening unified pods (FOUP's).
 10. The vertical batch furnace assembly according to claim 1, further comprising at least one cassette in-out port provided in a second wall bounding the cassette handling space, and a cassette handler mechanism configured to transfer wafer cassettes to and from the at least one cassette in-out port.
 11. The vertical batch furnace assembly according to claim 2, provided with a cassette handling mechanism comprising a first cassette handler and a second cassette handler, wherein the first cassette handler is configured to place a wafer cassette onto a cassette support surface of the carrousel stage which is in the first load/retrieve position and to retrieve the wafer cassette from the cassette support surface of the carrousel stage which is in the first load/retrieve position, and wherein the second cassette handler is configured to place a wafer cassette onto a cassette support surface of the carrousel stage which is in the second load/retrieve position and to retrieve the wafer cassette from the cassette support surface of the carrousel stage which is in the second load/retrieve position.
 12. The vertical batch furnace assembly according to claim 2, provided with a cassette handling mechanism comprising a first cassette handler and a second cassette handler, wherein both the first cassette handler and the second cassette handler are configured to place a wafer cassette onto a cassette support surface of the carrousel stage which is in the first load/retrieve position and to retrieve the wafer cassette from the cassette support surface of the carrousel stage which is in the first load/retrieve position, as well as to place a wafer cassette onto a cassette support surface of the carrousel stage which is in the second load/retrieve position and to retrieve the wafer cassette from the cassette support surface of the carrousel stage which is in the second load/retrieve position.
 13. The vertical batch furnace assembly according to claim 11, wherein the cassette handling space comprises a cassette storage having a plurality of cassette storage positions and configured to store a plurality of wafer cassettes, and wherein each cassette handler is provided with a cassette handler arm, and an elevator mechanism configured to reach cassette storage positions at different vertical heights within the cassette handling space.
 14. The vertical batch furnace assembly according to claim 13, wherein the elevator mechanism of the first cassette handler and the elevator mechanism of the second cassette handler are arranged on or adjacent respectively a third wall and a fourth wall which bound the cassette handling space on opposite ends.
 15. The vertical batch furnace assembly according to claim 1, wherein the at least one cassette carrousel is provided with a transfer mechanism configured to move a wafer cassette supported on a cassette support surface in the wafer transfer position towards and/or away from the first wall having the at least one wafer transfer opening.
 16. The vertical batch furnace assembly according claim 15, wherein for each wafer transfer opening the vertical batch furnace assembly comprises: a cassette door opener device arranged on the first wall and configured to engage a wafer cassette which is being moved towards the first wall by the transfer mechanism and to open a cassette door of said engaged wafer cassette.
 17. The vertical batch furnace assembly according to claim 16, wherein the vertical batch furnace assembly further comprises: a wafer handler positioned in the wafer handling space and configured to transfer wafers between a wafer cassette engaged to the cassette opener device and a wafer boat; a process chamber configured to process wafers accommodated in the wafer boat; and a wafer boat handling device positioned under the process chamber and adjacent the wafer handling space, wherein the wafer boat handling device is provided with the wafer boat in a wafer boat transfer position, and is configured to transport the wafer boat to the process chamber, and to the wafer boat transfer position. 